The invention relates to a method for the signal linearization of a gas sensor output signal.
The linearization of gas sensor output signals is used in many different areas. As an example this description uses an application in the medical field, specifically the use of gas sensors in the measurement of in- and exhaled gases.
In lung function diagnostics measurement of the diffusing capacity using carbon monoxide (DLCO) is a method that is based on the measurement of specific gas concentrations during in- and exhalation of a test gas by the patient. Commonly used DLCO gas mixes for this test type consist of 0.3% Carbon Monoxide (CO), 10% Helium (He), and 21% Oxygen (O2) with balance Nitrogen (N2). When a test is performed in a patient, this test gas is first inhaled by the patient, and then the patient performs a 10 seconds breath hold, followed by a normal exhalation. During the breath hold Helium is diluted, and the CO is diluted and absorbed into the blood. In order to compute the diffusing capacity the dilution of Helium and the absorption of CO have to be measured with high accuracy. For that purpose the inspiratory gas concentrations as well as the expiratory gas concentrations of Helium and CO must be measured using appropriate gas sensors. Normally separate gas sensors for CO and Helium are used. Since these gas sensors are often non-linear, (i.e. a linear increase in gas concentration is not equal to a linear increase in output signal of the sensor); the output of the sensor signal must be linearized. This is normally achieved by determining the non-linearity of the sensor during production. A sensor-individual or a standardized linearization is then applied to the output signal when the sensor is in use. This method, however, does not allow checking the linearization when the sensor is in use and this method can also not take into account changing characteristics of the gas sensor during its life time.
Currently linearization of an output signal of a gas sensor is normally performed using one of the following methods:    1. A fixed linearization curve is determined during the development of the gas sensor. That linearization curve is then applied to all gas sensors of that type. This method does not take into account that the characteristics of the gas sensor non-linearity may be different between individual gas sensors. It also does not take into account that the characteristics may change over time.    2. The linearization curve is determined on an individual base for each gas sensor that is produced. The linearization curve is determined using defined gas mixtures. This method does not take into account that the characteristics of the gas sensor may change over time.    3. The linearization curve of the gas sensor is determined during operation of the device using defined gas mixtures. In order to determine a linearization curve, normally at least three gas mixtures are used, i.e. gas mixtures at 0%, 50% and 100% of the output range of the gas sensor. When the sensor is in use this method requires at least one additional precision gas mix and therefore additional hardware.
As described above, the gas sensor linearization according to the prior art shows one or more disadvantages.